Thermoplastic resin compositions such as ABS resins, resins prepared by mixing polycarbonate resins and ABS resins (also referred to as polycarbonate/ABS resins), polyamide resins and mixed resins of polycarbonate resins and polyester resins (also referred to as polycarbonate/polyester resins), or resins prepared by reinforcing these thermoplastic resin compositions with inorganic fillers, are widely used as the materials for the casings used in mobile devices such as notebook and tablet type personal computers, mobile phones including smart phones, digital cameras and digital video cameras. The method used for producing these casings typically employs a method of molding the thermoplastic resin composition by injection molding, which enables the shape of the casing to be molded with relative freedom.
In recent years, demands being made on the casings used in mobile devices include further reductions in the thickness, the ability to withstand impacts and loads when stored inside a bag or the like, and the ability to be used in an uncoated state in order to lower costs. In order to satisfy these demands, the thermoplastic resin compositions used in the casings not only require superior levels of mechanical strength such as rigidity and impact resistance when formed as a molded article, but also require superior weld strength and flame retardancy, as well as favorable moldability during the molding process.
However, because thermoplastic resin compositions such as ABS resins, polycarbonate/ABS resins, polyamide resins and polycarbonate/polyester resins that have not been reinforced with an inorganic filler have low rigidity when formed into a molded article, they are unable to satisfy the demand for thinner casings. Further, polyamide resins exhibit high hygroscopicity, and after molding, the molded article tends to be prone to warping, dimensional changes or deterioration in the external appearance over time.
Accordingly, reinforced thermoplastic resin compositions of improved rigidity prepared by adding an inorganic filler such as glass fiber or carbon fiber to the above thermoplastic resin compositions are being investigated as thermoplastic resin compositions for use in casings.
However, although reinforced thermoplastic resin compositions containing an ABS resin, polycarbonate/ABS resin or polycarbonate/polyester resin as the main component exhibit superior rigidity when formed as a molded article and enable a reduction in the wall thickness of the casing, the weld strength and impact resistance when formed as a molded article are insufficient.
In particular, reinforced thermoplastic resin compositions containing a polycarbonate/polyester resin as the main component also exhibit poor thermal stability. Further, during the molding process, if the resin is held at high temperature inside the cylinder, then a transesterification between the polycarbonate resin and the polyester resin may generate a decomposition gas, increasing the likelihood of bubbles and external appearance defects known as silver streaks within the molded article. Furthermore, there is also a possibility that as a result of a decrease in the molecular weight of the polycarbonate resin due to this transesterification, the inherent impact resistance and heat resistance and the like of the polycarbonate resin may be lost. Moreover, another problem is that the viscosity of the polycarbonate resin may change during storage at high temperature, resulting in a loss of molding stability during injection molding, and the occurrence of filling faults (also known as short shots) or overfilling (also known as burrs) in the obtained molded articles.
On the other hand, although reinforced thermoplastic resin compositions containing a polyamide resin as the main component exhibit excellent weld strength when formed into molded articles, the aforementioned problems of warping, dimensional changes and deterioration in the external appearance cannot be resolved. These problems are due to moisture absorption by the molded article after molding, and are not problems that can be resolved by drying the molding material prior to molding.
The following compositions have been proposed as reinforced thermoplastic resin compositions that are capable of forming molded articles having excellent impact resistance.
(1) A reinforced thermoplastic resin composition containing an aromatic polycarbonate resin, a graft copolymer, glass fiber that has been surface-treated with a water-soluble polyurethane, a glycidyl ether unit-containing polymer, and a phosphate ester-based flame retardant (Patent Document 1).
(2) A reinforced thermoplastic resin composition containing an aromatic polycarbonate resin, a fibrous filler that has been surface-treated with a polyamide, and a lubricant having a carboxyl group (Patent Document 2).
The following compositions have been proposed as reinforced thermoplastic resin compositions that are capable of forming molded articles having excellent mechanical strength.
(3) A reinforced thermoplastic resin composition containing a polycarbonate resin, a rubber-containing polymer, and a carbon fiber that has been bundled with a nylon-based bundling agent (Patent Document 3).
(4) A reinforced thermoplastic resin composition containing a polycarbonate resin, a rubber-like polymer, a polyethylene terephthalate that has been subjected to a deactivation treatment with a polycondensation catalyst, a milled fiber, and an olefin-based polymer having a functional group that reacts with polycarbonate (Patent Document 4).